It is well known in the industrial heat treat furnace art to case harden metal workpieces by means of a process referred to as either ion processing or glow discharge technique or plasma arc heating. In the specifications herein, this technique will be simply referred to hereafter as ion processing.
In ion processing, the work to be heat treated is processed inside a vacuum vessel or electric furnace chamber which is evacuated with a mechanical pump and back filled to a partial pressure with a heat treating gas. If ion nitriding is to be performed, the gas contains ammonia and if ion carburizing is to be performed, a hydrocarbon such as methane is used. In addition, the work is generally sputtered clean initially by a hydrogen gas. In ion processing, the workpiece to be heat treated is electrically connected to the negative (cathode) terminal of an electrical power supply (either DC or AC rectified to DC) and the work is insulated from the vessel wall. The vessel wall is connected to the positive terminal (anode) of the power supply and to ground for safety reasons. When the power supply is energized, an electrical potential exists between the electrically conductive hearth with the workpieces sitting thereon and the vessel wall. The electrical potential causes the gases to ionize which then bombard the cathodic workpiece because of the vacuum (vacuum is defined herein to mean pressure less than atmospheric pressure) drawn in the vessel. As the gas ions bombard the workpiece some heat is imparted to the work (whether the plasma be viewed as cold, i.e. pulsed power supply, or not) and a glow about the workpiece occurs thus lending the process to the name "glow discharge". Heating can be caused solely by ion activity but conventional practice today is to use auxiliary heating not only to preheat the work but to bring the work to its heat treating temperature.
In the course of ion bombardment, metal sputters from the case and begins to coat the interior of the vessel. Metal is highly electrically conductive. In addition, depending on the gas used in the ion process (for example various metal plating processes ionize the gas to deposit metal onto the workpiece, i.e. germanium, boronizing, etc., or carbon from methane), such gas itself can deposit material which has a high dielectric constant. If the deposited, electrically conductive material builds up between the vessel casing and the hearth, electrical short circuiting and arcing will occur.
To avoid this problem, conventional practice follows the teaching disclosed in Gunther U.S. Pat. No. 4,246,434. Standard practice is to support a hearth by load support tubes fixed to the casing and secured to the underside of the rails. The load bearing tubes are encased within a ceramic shield to prevent the hearth rails from establishing electrical contact with the vessel casing. To prevent a buildup of electrically conductive material on the tubes, ceramic discs or spacers are provided along the tube length. The spacers establish tiny gaps which prevent the electrically conductive material produced in the ion process from establishing an electrically conductive conduit from hearth to tube.
Insofar as the feed-through is concerned (i.e. the negative terminal connection to the hearth), standard practice is disclosed in Verhoff U.S. Pat. No. 4,853,046. The power electrode extends through the furnace casing wall in an insulated manner and simply attaches to the hearth rail. The electrode can be insulated by being placed in a sheath within the vessel up to the hearth attachment point. An electrical interlock is then provided which disables the power supply so that the power supply cannot be energized until the vacuum pump is energized. This is generally regarded as a safe interlock. However, circuit failures can occur. For example, the vacuum pump could be sensed as on when a vacuum is not drawn in the furnace.
Apart from this, the prior art arrangements discussed above cannot work in the Indugas furnace design disclosed herein and in my prior patents. In my furnace a thin, imperforate shell member functions as the vacuum vessel. The shell thermally distorts. Any electrical connection requiring an opening through the shell will lead to premature shell failure. Also, any rigid connection of the hearth to fixed supports will cause the hearth to pitch and yaw and could vary the ion geometry.